Nanomaterial technology can show novel functions and characteristics which cannot be obtained from conventional materials and thus may be referred to as the most advanced fusion material technology which can be applied to various fields and industries.
For example, platinum nanocolloid is expected to be highly useful in cosmetics and food supplement fields. This is because the conventional materials regarded as having antioxidant properties can remove only a specific reactive oxygen species from seven kinds of reactive oxygen species in the body and do not act anymore if once they remove the reactive oxygen, whereas platinum nanocolloid can remove all reactive oxygen species and semipermanently act while remaining in the form of colloid in the body. Therefore, if platinum nanoparticles prepared without impurities using a colloid protective agent harmless to the human body are colloidized, the application thereof is not limited to catalyst, photoelectron, sensor, conductive device, and bio fields but can be expanded to medical and food supplement fields, and thus, the marketability thereof is expected to be highly increased.
In this nanotechnology, nanomaterials have been manufactured and applied as structures in various forms such as powder, tube, whisker, and thin film. Among these forms, powder and thin film forms are the most common. Techniques for preparing nanomaterials in the form of thin film have been practically accumulated for a long time, whereas techniques for preparing nanomaterials in the form of powder have been researched and developed but have not often been commercialized due to difficulties in reproducible production and storage.
In the case of a metal powder nanomaterial, as a size of powder is decreased, surface energy is increased due to an increase in specific surface area, and thus, the powder becomes unstable. Further, if metal has a critical size or less, the reactivity is increased, and thus, the metal can react with oxygen in air and causes spontaneous combustion. Therefore, an attempt to prepare highly active nanosized metal powder and stably use it is more desperately needed.
Further, along with a gradual spread of the fact that the industrial importance of metal nanoparticles is very high, the demand for a technique of mass-producing metal nanoparticles through an eco-friendly and economically competitive process has been greatly increased. Various methods for preparing nanoparticles have been developed, and can be roughly divided into vapor phase synthesis of synthesizing nanoparticles in a gaseous state and liquid phase synthesis including dissolution in a solution and growth of crystals. In general, the vapor phase synthesis has received attention as a method for mass-producing high-purity particles, but according to the vapor phase synthesis, primary particles produced during a reaction process are agglomerated to form clustered secondary particles, resulting in the production of strongly agglomerated particles, and thus, it is difficult to prepare nanoparticles with a uniform small size of 100 nm or less.
In this regard, methods of preparing nanoparticles through an aerosol method and evaporation/condensation in a gaseous phase, and the like have been widely developed as methods for synthesizing particles having a diameter of 100 nm or less. However, the vapor phase synthesis has not been widely used industrially because 1) it is difficult to mass-produce nanoparticles, 2) it is difficult to control a particle size and thus a separate process for particle separation is needed, 3) a process is performed at a high temperature in many cases, and 4) costs for preparing particles are generally high. As a method for resolving agglomeration of nanoparticles, a method for producing non-agglomerated nanoparticles by flame synthesis is disclosed in U.S. Pat. No. 5,498,446. According this method, when metal or ceramic particles are synthesized by heating a halogen-containing precursor in a reaction area of flame synthesis, a vaporized metal such as sodium (Na) is introduced, so that the metal or ceramic particles are coated with a by-product, i.e., sodium chloride (NaCl), and NaCl is dissolved using water or a solvent to separate particles of 100 nm or less from agglomerated nanoparticles. However, according to this method, a solvent is needed, and thus, there is a problem that a preparation process is complicated.
Meanwhile, in the case of the liquid phase synthesis, a preparation process is simple and economical, but the liquid phase synthesis has limitations in restricting a particle size to the range of nanometers and requires the use of a solvent and a reducing agent and thus may cause environmental problems. After the preparation of particles, additional processes for separating nanoparticles from a solution and purifying the nanoparticles are needed, and thus, the liquid phase synthesis has difficulty in mass-production. Further, in the case of using an organic solvent and a reducing agent together, volatile organic chemicals (VOCs) may be generated due to the use of the solvent and toxic wastewater has been inevitably generated. Furthermore, in most cases, reactants are used in a low weight ratio of from about 5 wt % to about 20 wt %, and thus, it is necessary to use very large reactor and auxiliary equipment relative to the amount of a product. Moreover, in order to obtain the product therefrom, an apparatus for separation and purification is needed, and thus, the preparation process may become complicated and preparation costs may be increased.
Meanwhile, hydrazine hydrate has been used as a reducing agent when metal nanoparticles are prepared from a metal compound dissolved in a solution. In the case of a solution process using a reducing agent such as hydrazine, there are drawbacks such that the productivity is not high due to the need of using a solvent and that an excessive amount of hydrazine needs to be used. Further, an excessive amount of an unused hydrazine solution may be harmful to the human body, and an additional process, such as a waste water treatment, for treating the unused hydrazine solution is needed.
Hydrazine (N2H4) has chemical properties similar to those of an ammonia (NH3) gas, but it is a clear liquid at room temperature and has melting and boiling points and density similar to those of water. As such, liquid hydrazines may cause contamination due to fire or rapid reaction with an ambient metal or material in case of their leakage, and most of liquid hydrazines contain a great amount of moisture and thus cannot be used in case of need of moistureless condition or have limitations in application due to side reactions caused by water.
As a method for reducing the above-described problems of liquid hydrazine, a method of preparing solid hydrazine salt through a reaction between liquid hydrazine and sulfuric acid or hydrochloric acid and thus using the solid hydrazine salt in substitution for liquid hydrazine has been suggested. Although various kinds of hydrazine salts have been developed, the application thereof has been very limited due to the low reactivity and the need for removing anions remaining after the reaction.
Meanwhile, U.S. Pat. No. 6,203,768 suggests a new method of producing nanoparticles through a mechanochemical method. According to this method, if a metal halide compound such as ferric chloride (FeCl3) and a metal such as sodium (Na) are put into a ball mill and reacted at a high temperature to form iron (Fe) nanoparticles surrounded by sodium chloride (NaCl), and then the sodium chloride (NaCl) is removed by dissolution or sublimation to obtain separated nanoparticles. However, according to this method, a process for removing a solvent is needed and it is difficult to obtain high-purity particles.
Although various methods such as a method using ultrasonic waves, a method using microemulsion, a cavitation processing, and high-energy ball milling have been reported as alternatives of the above-described two methods, these alternatives have not been generally used due to limitations in mass-producing metal nanoparticles and problems with preparation costs.